Newer designs and manufacturing techniques have driven electronic components to small dimensions and miniaturized many communication devices and systems. Unfortunately, antennas have not been reduced in size at a comparative level and often are one of the larger components used in a smaller communications device. It becomes increasingly important in communication applications to reduce not only antenna size, but also to design and manufacture a scalable size antenna having sufficient gain.
In current, everyday communications devices, many different types of patch antennas, loaded whips, copper springs (coils and pancakes) and dipoles are used in a variety of different ways. These antennas, however, are sometimes large and impractical for a specific application. Antennas having diverging electric currents may be called dipoles, those having curling electric currents may be loops, and dipole-loop hybrids may comprise the helix and spiral. While dipole antennas can be thin linear or “1 dimensional” in shape, loop antennas are at least 2 dimensional. Loop antennas can be a good fit for planar requirements.
Antennas can of course assume many geometric shapes. The Euclidian geometries are sometimes preferential for antennas as they convey optimizations known through the ages. For instance, line shaped dipoles may have the shortest distance between two points, and circular loop antennas may have the most enclosed area for the least circumference. So, both line and circle shapes may minimize antenna conductor length. Yet simple Euclidian antennas may not meet all needs, such as operation at small physical size relative wavelength and a self loading antenna structure may be needed. Cyclic curves may be advantaged for antennas and antenna arrays, yet cyclic antennas do not seem common in the prior art.
Simple flat or patch antennas can be manufactured at low costs and have been developed as antennas for the mobile communication field. The flat antenna or thin antenna is configured, for example, by disposing a patch conductor cut to a predetermined size over a grounded conductive plate through a dielectric material. This structure allows a nearly planar dipole antenna to be fabricated in a relatively simple structure. Such an antenna can be easily mounted to appliances, such as a printed circuit board (PCB).
Many applications, such as land mobile, may require thin planar antennas with vertical polarization when mounted in a horizontal plane. Such antennas can be planar monopoles, sometimes known as microstrip “patch” antennas. The advantages of these antennas including printed circuit manufacture, being mountable in low profile, and having high gain and efficiency have made them the antennas of choice in many applications. However, microstrip patch antennas typically are efficient only in a narrow frequency band. They are poorly shaped for wave expansion, such that microstrip antenna bandwidth is proportional to antenna thickness. Bandwidth can even approach zero with vanishing thickness (for example, see Munson, page 7-8 “Antenna Engineering Handbook”, 2nd ed., H. Jasik ed.). With a thin planar shape, the loop antenna may give more bandwidth for area than the microstrip patch.
The radiation pattern shapes of many small antennas are toroidal or a cos2 θ rose, similar to half wave dipoles. An isotropic radiation pattern is one that is spherical in shape, however, and it may be advantageous when antennas are not aimed or oriented. Small antennas of planar construction, having sufficiently isotropic radiation may be of considerable utility.
Body worn antennas may operate near human flesh which may have a relative permittivity of about 50 farads/meter and a conductivity of 1 mho/meter, which is somewhat akin to the properties of seawater. The flesh is lossy to electric currents I if an uninsulated antenna contacts skin, lossy to electric near fields E by dielectric heating, and lossy to magnetic near fields H by induction of eddy currents. In the design of body worn antennas it can be important to take these effects into account, as for instance dielectric heating is more pronounced at higher frequencies, induction of eddy currents more important at lower frequencies, and insulation may avoid conducted current losses.
Antenna frequency stability is another concern as drifted tuning may cause gain reduction. Few small antennas are unaffected by close proximity to the human body. Antennas transducing only one type of near field (E or H) might be advantageous, but they appear to be unknown.
Shielded body worn antennas may use a metal layer between the antenna and the body to reduce losses. Although the shield reduces body affects the shield itself has effects. The conductive shield must be of sufficient size and it may reduce efficiency and bandwidth: shield reflections can be akin to the image reversal of a mirror, e.g. 180 degrees out of phase causing signal cancellation. It may be preferential to avoid shields and ground planes in body worn antennas if possible.
U.S. Pat. No. 6,501,427 to Lilly et al. entitled “Tunable Patch Antenna” is directed to a patch antenna including a segmented patch and reed like MEMS switches on a substrate. Segments of the structure can be switched to reconfigure the antenna, providing a broad tunable bandwidth. Instantaneous bandwidth may be unaffected however.
U.S. Pat. No. 7,126,538 to Sampo entitled “Microstrip antenna” is directed to a microstrip antenna with a dielectric member disposed on a grounded conductive plate. A patch antenna element is disposed on the dielectric member.
U.S. Pat. No. 7,495,627 to Parsche entitled “Broadband Planar Dipole Antenna Structure And Associated Methods” describes a planar dipole-circular microstrip patch antenna with increased instantaneous gain bandwidth by polynomial tuning. Yet, other antenna types may be required for other needs, e.g. for horizontal rather than vertical polarization, or isotropic rather than omnidirectional radiation.
There is a need for a planar antenna that may be flexible and/or scalable as to frequency and provide adequate gain. Such an antenna may be desirable for use in patient wearable monitoring devices, for example, to provide telemetry of medical and vital information. There is also a need for an antenna having a radiation pattern sufficiently isotropic to avoid the need for product orientation, e.g. to avoid the need for antenna aiming as may be useful for radiolocation tags or tumbling satellites.